1. Field of the Invention
This invention relates to methods of fabricating a device having a desired non-planar surface or profile and devices produced thereby.
2. Background Art
Several MEMS applications require a non-planar surface topography, including high performance electrostatic actuators and optical components. For example, an electrostatically-driven parallel-plate actuator can provide a much larger vertical deflection with a low voltage if the drive electrode is curved. In this structure, a large force is created around the edges of the diaphragm (closest to electrode) to deflect it toward the electrode, while a large deflection (>10 μm) can be obtained in the middle of the diaphragm (farthest from electrode).
The optimal electrode shape for an electrostatic curved actuator should maximize diaphragm displacement while minimizing drive voltage and diaphragm bending stress. Legtenberg suggests that higher order curvature profiles require lower pull-in voltages in R. Legtenberg et al. , “Electrostatic Curved Electrode Actuators,” J. MICROELECTROMECH. SYST. , Vol. 6, No. 3,pp. 257-265, 1997.
Devices, such as microactuators for fluid control application, have utilized in-plane curved electrode configuration in order to achieve large beam displacement at tip. F. Sherman et al. , “In-Plane Microactuator for Fluid Control Application,” PROC. MEMS '98, pp. 454-459, 1998.
Micropumps with diaphragm driven by out-of-plane curved electrode have been used to achieve high air pumping rate. C. Cabuz et al. , “The Dual Diaphragm Pump,” PROC. MEMS '01, pp. 519-522, 2001.
However, curved electrodes cannot be batch-fabricated for those devices since it is difficult to batch fabricate an out-of-plane curved surface on the wafers.
Past attempts at making out-of-plane curved surfaces utilized gray-scale optical masks to create photoresist with a prism profile. C. Gimkiewicz et al. , APPLIED OPTICS, Vol. 38, No. 14, pp. 2986-2990, 1999. This pattern is then transferred onto Si using RIE. However, this technique requires well controlled photolithography and stable RIE selectivity between photoresist and Si. More importantly, gray-scale optical masks are extremely expensive, especially when a very fine gray-scale is required.
Silicon DRIE is well known for its feature size-dependent etch rate when the aspect-ratio is >2,as illustrated by H. Jansen et al., MICROELECTRONIC ENGN. , Vol. 35, pp. 45-50, 1997. In most cases, this is undesirable since it results in uneven etch depth for different openings.